1. Field of the Invention
The present invention relates to a four-wheel steering apparatus for vehicles and, more particularly, to a four-wheel steering apparatus for vehicles adapted to change a turning ratio of the rear wheels to the front wheels.
2. Description of the Prior Art
A four-wheel steering apparatus for a vehicle is known to turn the rear wheels in association with the front wheels in response to the operation of a steering wheel by a operator that includes a front wheel turning mechanism and an intercoupled rear wheel turning mechanism such that a steering angle of the steering wheel is arranged to be transmitted through both turning mechanisms to the front wheels and the rear wheels as a turning angle.
In such known four-wheel steering apparatus, a turning ratio of the rear wheels to the front wheels (that is, a ratio of a turning angle of the rear wheels to that of the front wheels) is not always constant, and may be commonly variable on the basis of a predetermined turning ratio characteristics. The turning ratio characteristics is set using a running state of the vehicle as a parameter. The parameters may include, for instance, vehicle speed, the steering angle of a steering wheel, and a transverse G (an acceleration in the transverse direction) acting on the vehicle.
The direction in which the rear wheels are turned may be determined by a so-called same phase direction and a so-called reverse phase direction.
In such four-wheel steering apparatus, when vehicle speed is low or a steering angle of the steering wheel is large, the rear wheels are turned in reverse phase direction so as to obtain an improved turning rate. Under a high vehicle speed or a small steering angle, the rear wheels are turned in same phase direction so as to obtain an improved maneuverability of the vehicle.
It has further been proposed in such four-wheel steering apparatus to provide a power-assisted steering mechanism for steering the rear wheels. For example, Japanese Utility Model Public Disclosure No. 57087/1986 discloses a vehicle four-wheel steering apparatus having a front power-assisted steering mechanism for steering front wheels, and a rear power-assisted steering mechanism for steering rear wheels. Hydraulic oil passages or conduits connect a hydraulic pump with the front and rear power-assisted steering mechanisms to supply oil to the power cylinders of the power-assisted steering mechanisms. The hydraulic pump is driven by the vehicle engine.
On the other hand it has further been proposed in such a four-wheel steering apparatus to provide a fail-safe system. Japanese Patent Public Disclosure No. 77570/1986 discloses a vehicle four-wheel steering apparatus having a return spring in the rear power-assisted steering mechanism. The return spring is provided for returning a power cylinder to its neutral position when oil pressure applied to the rear power-assisted steering mechanism is drained from an on-off valve. The on-off valve is actuated to the open position for drain of the oil pressure when the electric system fails.
In the four-wheel steering apparatus described above, the on-off valve is actuated to the closed position by electric force or power applied via the ignition switch, so that when the ignition switch is opened, the on-off valve moves to the open position. Just to turn off the ignition switch, will not stop the engine in a short time because of the force of inertia, so that the hydraulic pump driven by the engine will generate oil pressure during this short time.
If the on-off valve is positioned at the open position at the same time with the ignition switch turned off, oil pressure generated by the hydraulic pump will drain through the on-off valve abruptly. In this condition, the oil will flow at high velocity in the conduit and will lose velocity energy at a bending portion of the conduit generating a crashing sound.